There are two complementary strategies for obtaining basic information
about radio sources from measurements of structure. One can
extrapolate conclusions drawn from detailed measurements of individual
source morphologies to the whole population of radio sources, or one
can try to infer properties of radio sources from statistical analyses
of well-defined samples. Highly detailed measurements can only be made
for a typical source, e.g. the closest (Cygnus A,
Virgo A, Centaurus A) or the largest (3C236, DA 240, NGC 315, NGC 6251). On the other
hand, the statistical approach limits the range of properties one can
investigate. And even when samples are chosen with the greatest of
care, selection effects that are difficult to take into account often
remain.